The limits of luminance for reading comfort are known based upon experimentation to fall within the range of 10 to 100 milliLamberts (mL). This is the approximate equivalent luminance of white paper under 10 to 100 foot candles of illumination. In order to limit the amount of light impinging on the eye to the 10 to 100 mL range, there exists a need for spectacle lenses that remain maximally transmissive below 100 mL and become increasingly dense above the 100 mL illumination level. Previous attempts to design a lens for this purpose have lead to the development of photochromic (also called phototropic or identified under the trademark PHOTOGRAY) glass that darkens when exposed to light but regains its original transparency a period of time after the light is removed. Photochromic glass has a relatively slow response time. At least 60 seconds are required for photochromic glass to darken to 75% of its maximum density with full darkening requiring at least 5 minutes. Even when fully darkened, the density of photochromic glass is less than that of most conventional sun-glasses. An even more significant drawback of photochromic glass is that recovery to initial transparency is very slow; recovery is only 50% in 5 minutes to 75% in 20 minutes. The sight of an individual entering a dark room from high intensity ambient lighting can thus be temporarily blocked during the recovery period. Furthermore, maximum density of the prior art device is inversely proportional to temperature and it is mainly sensitive to wavelengths outside the range of human vision.
There exists a present need, therefore, for a lens that approaches the characteristics of "ideal sunglasses", that is, one which has fast response time over a wide range of optical densities. The need goes beyond that of simple comfort or convenience. Strict control of light levels incident on the eye is often required for physiologic reasons, i.e., tape-toretinal degenerations, macular disease or for therapeutic reasons, i.e., ocular inflammatory states, post-operative cataract surgery or sector iridectomies.
One object of the present invention, therefore, is to provide a variable density lens that has a faster response time than the prior art photochromic lens.
Another object is to provide a fast response time variable density lens that is controlled automatically in response to ambient light intensity in the visible range.
Another object is to provide a spectacle lens having a density that is continuously variable within a wide density range in response to ambient light intensity and has a short response time to change of ambient light intensity.